1. Field of the Invention
The present invention relates to an image forming apparatus and more particularly to an image forming apparatus for forming an image by electrophotography.
2. Description of the Prior Art
There have so far been proposed many types of such multicolor image forming apparatus forming images by means of electrophotography. These are broadly classified into the following groups. The first group is such that wherein electrostatic latent images for separated colors are formed and developed in succession for each of separated colors on a single photosensitive member, and all the colors are superimposed on the photosensitive member or a developed toner image is transferred therefrom to a transfer material each time the development takes place, and thereby, all the colors are superimposed on the transfer material. The second group is such that wherein a plurality of photosensitive members corresponding in number to the number of colors are provided and the toner images of the separated colors are simultaneously formed on these photosensitive members and then these images are transferred one by one to a transfer material and thereby a multicolor image is provided. In the latter case, it is advantageous that high-speed processing is made possible since toner images of multiple colors are simultaneously formed on the photosensitive members, but since a plurality of sets of photosensitive members, exposure means, and so on are required, the apparatus becomes more complex, larger in size, and more expensive, and hence, less practical. And, in either type of the above multicolor image forming apparatus, there is a great disadvantage that it is difficult to register the color images at the time of superimposing them, and therefore, it is impossible to completely eliminate the shear in the superimposition of colors.
To thoroughly solve these problems, the present inventor earlier invented an apparatus capable of forming a multicolor image by a single image exposure on a photosensitive member. The apparatus, using an electroconductive member, a photoconductive layer, a photosensitive member having a layer including a plurality of kinds of filters, forms a multicolor image as described below. That is, by providing the surface of the above mentioned photosensitive member with electric charges and an image exposure, an image (a primary latent image) is formed by variations in charge density on the boundary surface between an insulating layer and the photoconductive layer, and then by providing the image carrying surface with a whole surface exposure by specific light, that is, the beam of light passing through only one filter portion of the above mentioned plurality of kinds of filters, a potential pattern (a secondary latent image) is formed on the photosensitive member at the portion of the filter, and further by developing the potential pattern with a developing device containing a toner of a specific color, a single color toner image is formed. In succession thereto, by making another whole surface exposure by the beam of light passing through the filter portion different from that previously used and developing with another developing device containing a toner of different color from that used before, a toner image of the second color is formed on the photosensitive member. Thereafter, the whole surface exposure and development are repeated a required number of times. As a result, toners various colors are attached to various filter portions on the photosensitive member, and thus, a multicolor image is formed thereon (refer to Japanese Patent Application Nos. 59-83096 and 59-187044). According to this type of multicolor image forming apparatus, the image exposure is made only once, and therefore, there is really no possibility of occurrence of the shear in the colors superimposed.
In the above described multicolor image forming apparatus, reproduction of colors is carried out by not superimposing different colors on the same portion, namely, it is made through the so-called additive process. That is, the reproduction of black color by toners of three--yellow, magenta, and cyan--colors, for example, is performed by arranging these toners so that none of them is placed on another on a recording material and the black color is represented by composite reflected beams of light of these color components. High fidelity in color reproduction is obtained and the problem of the shear in the color images is solved by the present method, but in order to obtain good color balance, the potential contrast of the above mentioned potential pattern before development, which decides the attached amount of the toner, must be secured for each of the toners.
It has been known to produce a picture image of different colors from the original, that is, to make so-called color conversion. In such a case, the color conversion is achieved by changing the combination of the secondary latent image formed at a specific filter portion by the whole surface exposure by light of a specific color with the developing device containing a specific toner.
However, when the above mentioned combination is changed, the intervals of time between the image exposure, whole surface exposure, and development are changed from those when the color conversion is not practiced, and thereby, the color balance is largely affected.
The reason therefor is that, since a panchromatic photoconductive layer, having a sensitivity extended to the range of high wavelengths, exhibits in general no high electric resistance even at a dark place, the distributed electric charges on the boundary surface of the photosensitive layer trapped thereby vary with time due to neutralization by injection of electric charges from the substrate or escape of the electric charges to the conductive substrate, and so, the potential contrast of the potential pattern changes with the change of the intervals of time between the image exposure and the whole surface exposure. Occurrence of such a change is called dark decay. The degree of the dark decay depends upon the kinds of the photoconductive layers.
In the previously described conventional multicolor image formation, the image exposure, whole surface exposure, and development for one color only are performed in one rotation of the photosensitive member, and so, the above described problem does not occur even in practicing the color conversion. However, in the case of the multicolor image forming apparatus of the above described Japanese Patent Application Nos. 59-83096 and 59-187044, the primary latent image is formed for all colors at one time of image exposure, and therefore, the problem as described above occurs when the color conversion is practiced. So far, there has been given no consideration to the described problem.